In optical communication systems, the transmission speed of backbone optical transmission lines is advancing from 10 Gb/s to 40 Gb/s, accompanying the recent rapid increase in transmission capacity. In order to realize transmission speeds of 40 Gb/s, various types of optical modulation systems are proposed, and amongst these, a differential phase shift keying (DPSK) or a differential quadrature phase shift keying (DQPSK) serving as optical phase modulated systems are considered as most likely. In particular, because the DQPSK comparatively alleviates the demand for high speed in electrical devices, the adjustment of optical dispersion, and polarization mode dispersion, the DQPSK has become a strong candidate for optical modulation systems.
A configuration example of an optical communication system of such a DQPSK optical modulation system is shown in FIG. 5 though FIG. 7. FIG. 5 shows a configuration example of a point-to-point system which does not have a redundant system, while FIG. 6 shows a configuration example of a point-to-point system which is provided with a redundant system for improving reliability. Furthermore, FIG. 7 shows a configuration example of an optical unidirectional path switched ring (OUPSR) system combined with a reconfigurable optical add-drop multiplexer (ROADM) in consideration of application to a ring network.
In these optical communication systems, transponder devices TP are used, that inputs therein an STM 256/OC768 (40 Gb/s) signal, as a client signal, and convert this signal into an optical transport network (OTN) frame format, and transmit an OTU 3 (43 Gb/s) signal to a backbone optical transmission line. Also, the transponder devices TP receive the OTU 3 (43 Gb/s) signal from the backbone optical transmission line and subject this signal to inverse transformation of the OTN frame format, and output the client signal, as the STM 256/OC768 (40 Gb/s) signal. A configuration example of these transponder devices TP is shown in FIG. 8. FIG. 8A shows an example of a transponder device TP used in the system that does not have a redundant system shown in FIG. 5, while FIG. 8B shows an example of a transponder device TP used in the system provided with a redundant system shown in FIG. 6 and FIG. 7.
The transponder device TP includes: a very short reach (VSR) module 1 (40 G) which is an interface on the client side; a narrow band (NB) module 2 (43 G) that is an interface on the backbone optical transmission line (network) side; and an OTN framer 3 (40 G) that converts an output signal of the VSR module 1 into a signal of OTN frame format and inputs this converted signal to the NB module 2, and conversely inverse transforms a signal of OTN frame format output from the NB module 2, and inputs this inverse transformed signal to the VSR module 1. Furthermore, as shown in FIG. 8B, in the transponder device TP for the optical transmission line provided with a redundant system, there is provided an optical coupler 4 and an optical switch 5 between the NB module 2 and the plurality of optical transmission lines. The optical coupler 4 branches the OTU 3 signal output from the NB module 2 and transmits these branched signals to both of the active/standby optical transmission lines, respectively. The optical switch 5 selects one of the OTU 3 signals received from both of the active/standby optical transmission lines, and outputs this selected signal to the NB module 2.
FIG. 9 shows details of the NB module 2. The inside of the NB module 2 can be largely divided into two parts, namely an optical transmitter 10 and an optical receiver 20.
In the optical transmitter 10, a 16:1 serializer 11 multiplexes the sixteen 2.69 Gb/s signals input in parallel from the OTN framer 3 to make a signal of 43 Gb/s, and then a 1:2 demultiplexer 12 separates this output signal from the 16:1 serializer 11 into two 21.5 Gb/s data signals and one 21.5 GHz clock signal. The respective data signals are input to drivers 13a and 13b of a DQPSK LN module (LiNbO3 module) 13, and the clock signal is input to a driver 14a of a return to zero (RZ) LN module 14. An output light of a tunable laser diode (LD) 15 serving as a variable length laser, is phase modulated in accordance with the 21.5 Gb/s data signals in the LN module 13, and then intensity modulated in accordance with the 21.5 GHz clock signal in the LN module 14, so that an OTU 3 signal of the RZ-DQPSK modulation type is transmitted to the optical transmission line.
In the optical receiver 20, a delay interference section 21 receives the RZ-DQPSK type OTU 3 signal from the optical transmission line. In the delay interference section 21, the received signal is branched into two branches, namely an A branch (I or Q) and a B branch (Q or I), and for each, optical phase adjustment of the branch signal is executed. After this, the respective branch signals are subjected to balanced detection in a balanced optical detecting section 22 provided with two twin photodiodes, and current/voltage conversion and the like is executed in a trans-impedance amplifier and a limiting amplifier within a data regenerating section 23. The respective reception signals of 21.5 Gb/s output from the data regenerating section 23 are multiplexed in a 2:1 multiplexer 24, and the 43 Gb/s signal due to the multiplexing is separated into sixteen 2.69 Gb/s data signals, by a 1:16 deserializer 25. The 2:1 multiplexer 24 at this time is operated in accordance with a 21.5 GHz clock signal from a clock recovery circuit 26 that regenerates clock signal contained in the reception signal.
A detailed configuration example of the optical receiver 20 is shown in FIG. 10. The optical receiver 20 of this conventional example is disclosed for example in Japanese Unexamined Patent Publication No. 2007-020138 (in particular refer to FIG. 5).
The delay interference section 21 is provided with an A branch and a B branch, and in each of these branches is arranged for example delay interferometers 21a and 21b that use Mach-Zehnder interferometers, and heaters 21c and 21d serving as temperature control devices for the delay interferometers 21a and 21b. The phase shift amount of the delay interferometers 21a and 21b is changed corresponding to temperature change, and for example, the delay interferometers 21a and 21b have a characteristic where the phase shift amount increases with a rise in temperature. However they are not limited to this, and a device that adjusts the phase shift amount using a voltage change or the like is also suitable. The heaters 21c and 21d function as phase shift amount control devices that control the phase shift amount of the delay interferometers 21a and the 21b in accordance with an optical phase control value. A Peltier element 21e is provided as an ambient temperature control device for maintaining the ambient temperature of the delay interferometers 21a and 21b at a predetermined temperature. In the case where it is possible to control the phase shift amount of the delay interferometers 21a and 21b to a target value without the Peltier element 21e, the ambient temperature control element such as the Peltier element 21e can be omitted. These heaters 21c and 21d, and the Peltier element 21e are controlled by a temperature controller 21f provided with a thermometer for measuring the ambient temperature, and a DA converter (DAC), in accordance with a central processing unit (CPU) 30 serving as a control section.
As described above, in the case of the optical receiver that adjusts the optical phase with the temperature control information of the delay interferometer as the optical phase control value, there is a problem in an operation permitting a signal to be communicated (i.e., signal communication operation) in the not yet signal communicated state, for example, the operation performed when transmitting initial signal at system start-up or recovering after occurrence of signal interruption in the optical transmission line. The problem is that several minutes to several tens of minutes is required until the signal is actually communicated in the operation. That is to say, as shown in FIG. 11, in the optical phase control method in the above described optical receiver, when the signal communication operation is performed, the optical phase control is executed that starts control from the initial value (temperature minimum value) of the optical phase control value, and raises the optical phase control value at a constant control gradient of for example 1° C. per minute, and searches for the signal communication point. Therefore, in order to raise a temperature of the heater to a temperature to set the phase shift amount of the delay interferometer that communicates the signal, several minutes to several tens of minutes is required.
Also, in the optical communication system provided with a plurality of optical transmission lines serving as the redundant system, as shown in FIG. 6 or FIG. 7, the above-mentioned delay of the optical phase control might occur in the case of performing switching from the active optical transmission line to the standby optical transmission line. That is to say, since there are not many situations where the length of the optical transmission lines for active and standby are the same, then when the optical transmission line is switched, optical phase adjustment in the optical receiver must be performed. Also when switching the optical transmission lines, in the conventional optical phase control method, control starts from the initial value of the optical phase control value. Therefore a discrepancy the same as at the time of the aforementioned signal communication operation arises.